U.S. patent application Ser. No. 13/175,681 (“the '681 patent application”), filed Jul. 1, 2011, entitled, “Overlay System With Digital Optical Transmitter For Digitized Narrowcast Signals,” which is incorporated herein by reference in its entirety, discloses various implementations of improved cable-based overlay systems used to deliver high-definition digital entertainment and telecommunications such as video, voice, and high-speed Internet services from a headend to subscribers. Implementations of the improved overlay systems disclosed in the '681 patent application can use digitizers in the headend to re-digitize narrowcast analog signals after they have been QAM modulated and upconverted to radio frequency (RF) frequencies.
The implementations of the improved overlay system disclosed in the '681 patent application are not limited to any particular digitizer. For example, to re-digitize narrowcast analog signals after they have been QAM modulated and upconverted to RF frequencies, a bandpass filter can filter the narrowcast RF analog signals to filter out signals outside a desired frequency range, and the resulting signal can be converted to a digital signal by an A/D converter. The digital signal can be filtered further by another bandpass filter, downconverted to a baseband signal by a digital mixer, and low pass filtered by filter to produce a digitized signal. Then the digitized narrowcast RF signals can be converted to narrowcast RF optically modulated signals by digital narrowcast transmitters and transmitted over the overlay system to fiber nodes. Within each fiber node the narrowcast signals can be converted back to analog RF modulated signals by a converter, e.g., including an D/A converter.
The cost and availability of A/D converters used to implement the digitizers for implementations of the improved overlay system disclosed in the '681 patent application can be a limiting factor in such implementation. That is, for example, narrowcast RF analog signals input to the digitizers can have a 480 MHz-wide frequency spectrum with an upper limit of 1002 MHz. Accordingly, in such examples, the digitizer can imply an ultra high speed A/D converter having a sampling frequency of at least 2.1 GHz, for example, and a resolution of 12-bits or more.